Szewczak A A, Moore P B, Chang Y L, Wool I G
Department of Chemistry, Yale University, New Haven, CT 06511.
Proc Natl Acad Sci U S A. 1993 Oct 15;90(20):9581-5. doi: 10.1073/pnas.90.20.9581.
The sarcin/ricin loop is a highly conserved sequence found in the RNA of all large ribosomal subunits. The cytotoxins alpha-sarcin and ricin both inactivate ribosomes by cleaving a single bond in that loop. Once it has been attacked, ribosomes no longer interact with elongation factors properly, and translation stops. We have determined the conformation of the sarcin/ricin loop by multinuclear NMR spectroscopy using E73, a 29-nucleotide RNA that has the sarcin/ricin loop sequence and that is sensitive to both toxins in vitro. The sarcin/ricin loop has a compact structure that contains several purine.purine base pairs, a GAGA tetraloop, and a bulged guanosine adjacent to a reverse Hoogsteen A.U pair. It is stabilized by an unusual set of cross-strand base-stacking interactions and imino proton to phosphate oxygen hydrogen bonds. In addition to having interesting structural features, this model explains many of the biochemical observations made about the loop's structure and its reactivity with cytotoxins, and it sheds light on the loop's interactions with elongation factors.
帚曲霉素/蓖麻毒素环是在所有大核糖体亚基的RNA中发现的高度保守序列。细胞毒素α-帚曲霉素和蓖麻毒素都通过切割该环中的一个单键使核糖体失活。一旦受到攻击，核糖体就不再与延伸因子正常相互作用，翻译就会停止。我们使用E73（一种含有帚曲霉素/蓖麻毒素环序列且在体外对这两种毒素都敏感的29个核苷酸的RNA）通过多核核磁共振光谱法确定了帚曲霉素/蓖麻毒素环的构象。帚曲霉素/蓖麻毒素环具有紧凑的结构，包含几个嘌呤-嘌呤碱基对、一个GAGA四环和一个与反向Hoogsteen A-U对相邻的凸起鸟苷。它通过一组不寻常的跨链碱基堆积相互作用和亚氨基质子与磷酸氧氢键得以稳定。除了具有有趣的结构特征外，该模型还解释了许多关于该环的结构及其与细胞毒素反应性的生化观察结果，并揭示了该环与延伸因子的相互作用。
